The present invention relates to a process for preparing low-viscosity reaction products of polyisocyanates containing activated groups ethylenically unsaturated which react, with polymerization, on exposure to actinic radiation, to a process for preparing them and also to their use in coating materials.
The curing of coating systems which carry activated double bonds by actinic radiation, such as UV light, IR radiation or else electron beams, is known and is established in industry. It is one of the most rapid curing methods in coating technology. Coating compositions based on this principle are thus referred to as radiation- or actinically curing or curable systems.
Because of the environmental and economic requirements imposed on modern coating systems, that they should use as little organic solvents as possible, or none at all, for adjusting the viscosity, there is a desire to use coatings raw materials which are already of low viscosity. Known for this purpose for a long time have been polyisocyanates with an allophanate structure as are described, inter alia, in EP-A 0 682 012.
In industry these substances are prepared by reacting a monohydric or polyhydric alcohol with large amounts of excess aliphatic and/or cycloaliphatic diisocyanate (cf. GB-A 994 890, EP-A 0 000 194 or EP-A 0 712 840). This is followed by removal of unreacted diisocyanate by means of distillation under reduced pressure. According to DE-A 198 60 041 this procedure can also be carried out with OH-functional compounds having activated double bonds, such as hydroxyalkyl acrylates, although difficulties occur in relation to the preparation of particularly low-monomer-content products. Since the distillation step has to take place at temperatures up to 135° C., in order to be able to lower the residual isocyanate content sufficiently (<0.5% by weight of residual monomer), it is possible for double bonds to react, with polymerization, under thermal initiation, even during the purification process, meaning that ideal products are no longer obtained.
The preparation of low-monomer-content, allophanate-containing, polyurethane-based, radiation-curing binders is described in EP-A 0 867 457 and US-A 5 739 251. These binders, however, do not carry activated double bonds but instead carry unreactive allyl ether groups (structure R—O—CH2—CH═CH2). It is therefore necessary to add reactive diluents (low molecular weight esters of acrylic acid), which introduce the required UV reactivity.
There has also been no paucity of attempts to prepare allophanates indirectly, from isocyanate derivatives, other than urethanes and isocyanates. For instance, EP-A 0 825 211 describes a process for synthesizing allophanate structures from oxadiazinetriones, although no mention is made there of radiation-curing derivatives containing activated double bonds. Transposition to the particular circumstances of radiation-curing systems is described in WO 2004/033522.
Another route is the opening of uretidiones (cf. Proceedings of the International Waterborne, High-Solids, and Powder Coatings Symposium 2001, 28th, 405-419, and also US-A 2003 0153713) to give allophanate structures, which have also been already successfully transposed to radiation-curing systems (WO 2005/092942).
Both routes require high-grade base materials as starting material and lead only to an allophanate product which is rich in by-products.
U.S. Pat. No. 5,777,024 describes the preparation of low-viscosity radiation-curing allophanates by reacting hydroxy-functional monomers which carry activated double bonds with isocyanate groups of allophanate-modified isocyanurate polyisocyanates. The radicals attached via the allophanate groups are saturated, and so any possible higher functionality is foregone.
EP-B 694 531 describes a multi-stage process for preparing hydrophilicized allophanates containing radiation-curing groups. In that case, however, first an NCO- and acrylate-functional urethane is prepared, which is hydrophilicized and subsequently allophanatized following addition of a further NCO- and acrylate-functional urethane. As the process temperature for the allophanatization, temperatures of 100-110° C. are specified.
Finally, a process was presented in EP-A 1 645 582 that leads, starting from simple diisocyanates, through reaction with hydroxy-functional acrylates, without distillation of the products, to low-viscosity allophanates. A disadvantage of this process, however, is that a satisfactory reaction rate is achievable only with ammonium salts that are difficult to obtain. Additionally, the viscosities of the products described are not as low as, for example, the viscosities of the allophanates obtainable by the process described in EP-A 0 825 211.
It was therefore the object of the present invention to provide a process with which allophanates of lower viscosity—as described in EP-A 1 645 582—and crosslinkable by actinic radiation (radiation-curing allophanates) are provided which are obtainable with more easily accessible base materials.
Starting from EP-A 1 645 582 it has now been found that particularly low-viscosity radiation-curing allophanates of this kind are preparable by using, as the catalyst, zinc octoate in combination with a tertiary amine. EP-A 1 645 582 describes only the use of only one component as a catalyst. EP-A 1 645 582 also does not describe that through the combination of the two compounds it is possible to lower the viscosity by a multiple, despite the fact that EP-A 1 645 582 likewise deals with the problem of low-viscosity allophanates.